Max Planck Institute of Quantum Optics

The work of the Max Planck Institute of Quantum Optics focuses on investigating the quantum world with laser light. The physicists employ complex facilities comprising many optical components, such as mirrors and lenses, to trap and manipulate systems of quantum particles right down to individual atoms or molecules. They are thus laying the foundations for the quantum computers of the future, at the same time gaining insight into new types and exotic states of quantum matter. By generating ultra-short and high-intensity flashes of light the scientists can observe and control the motion of electrons in atoms. These experiments pave the way for extremely fast electronics and new types of radiation sources for medical diagnostics and therapy.

Researchers at the Max Planck Institute of Quantum Optics (MPQ) and the Department of Physics of Harvard University, Cambridge, USA, will collaborate in the new Max Planck Harvard Research Center for Quantum Optics
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Modern quantum physics holds quite a few promises in store: quantum computers and simulators will be able to trawl through huge quantities of data at lightning speed, accelerate the development of new drugs or facilitate the search for materials for, say, energy engineering. The research being carried out by Ignacio Cirac, Director at the Max Planck Institute of Quantum Optics in Garching, is helping to fulfill these promises.

Gravitational waves are some of the most spectacular predictions of the 1915 general theory of relativity. However, it wasn’t until half a century later that physicist Joseph Weber attempted to track them down. In the early 1970s, Max Planck scientists also began working in this research field, and developed second-generation detectors. The groundwork laid by these pioneers meant the waves in space-time ceased to be just figments of the imagination: in September 2015 they were finally detected.

Electrons hold the world together. When chemical reactions yield new substances, they play a leading role. And in electronics, too, they are the protagonists. Together with his colleagues,
Ferenc Krausz, Director of the Max Planck Institute of Quantum Optics in Garching, photographs the rapid movements of electrons with attosecond flashes, creating the basis for new technological developments.

Physicists can solve many puzzles by taking more accurate and careful measurements. Randolf Pohl and his colleagues at the Max Planck Institute of Quantum Optics in Garching, however, actually created a new problem with their precise measurements of the proton radius, because the value they measured differs significantly from the value previously considered to be valid. The difference could point to gaps in physicists’ picture of matter.

Quantum physics effects not only bear witness to the exotic nature of the microworld; they also facilitate completely new approaches, for instance in data processing. To better understand them, the team working with Immanuel Bloch, Director at the Max Planck Institute of Quantum Optics in Garching, is using atoms in optical lattices to simulate quantum matter.

Electrons don’t have much in common with basketballs, apart from the fact that they are often portrayed as having the shape of a ball. Nevertheless, Peter Hommelhoff is as adept a player with one as he is with the other. In his experiments at the Max Planck Institute of Quantum Optics in Garching, where he heads a Max Planck research group, he has achieved a new level of control over these elementary particles.

Single atoms can’t be grasped through everyday experience: even a drop of water or a microorganism is made up of countless numbers of them. But Gerhard Rempe, Director at the Max Planck Institute for Quantum Optics in Garching, uses single atoms to study the interaction between light and matter at the most elementary level. The work that he and his team perform is creating the foundations for a future quantum internet.

A particle that exists in two locations at once – this is found only in the quantum world. When physicists study phenomena like this, they learn quite a bit about the mysterious universe of the very small.

Quantum cryptography makes secure communication possible today, but with existing technology is limited to less than 100 km. A quantum repeater could overcome this limitation in principle but has not been built yet. We follow two different experimental approaches toward a photon-photon quantum gate, which is an essential component needed for a quantum repeater.
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Superconductors provide energy transport without loss due to vanishing electrical resistance below a certain temperature. Concerning technological applications the so-called high-temperature superconductors are particularly interesting as their critical temperature can be reached by cooling with liquid nitrogen. However, up to now the mechanisms underlying this phenomenon are not fully understood. Quantum simulators as realized by scientists from the “Quantum Many-Body Systems Division” promise to bring more insight.
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Complex quantum systems can order in a wide range of different ways. While order in conventional matter can be explained from local properties of the system, strongly correlated systems exhibit so-called topological order, where the quantum correlations of the system, termed entanglement, organize globally. Yet, methods from Quantum Information Theory allow model the entanglement structure of these systems locally and thus open up a plethora of applications in the study and classification of topologically ordered systems.
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Molecules possess fascinating characteristics like a wealth of internal states, an exceptional interaction or interesting chemical properties. At extremely low temperatures, quantum effects dominate which opens perspectives for simulation of complex quantum systems or the production of new quantum phases of matter. To experimentally achieve these temperatures, new methods for trapping and cooling molecules and for manipulating their internal states have to be developed.
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Mesons are made of two quarks; they are highly unstable forms of matter, surviving for around a billionth of a second or less. Since 2014, researchers at the MPQ have used particle accelerators located at the Paul Scherrer Institute to synthesize artificial atoms that contain these mesons: here a pion and an electron orbit the nucleus of helium atoms. For the first time, the pion mass was determined by spectroscopy with infrared laser light with a very high precision. Likewise the mass of the antiproton relative to the mass of the electron was measured with high precision at CERN.
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Complex quantum systems can display a variety of surprising phenomena, such as the fractional quantum Hall effect. The fundamental and technological interest in this effect motivates the question: "Are there different possibilities for realizing the effect?" Researchers at MPI of Quantum Optics and Universidad Autónoma de Madrid have proposed a new way to obtain fractional quantum Hall physics in lattice systems, and they have developed a recipe to implement a fractional quantum Hall state in ultracold atoms in optical lattices.
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Tracing and controlling the dynamics of electrons inside atoms, molecules or solids as they occur in real time resides at the forefront of modern science. This report discusses how recent advances in the synthesis and control of light field set the stage to quantum controlling electrons on their native (attosecond) time scale. These developments hold the promise for new, fundamental insights into the inner workings of the microcosm and form the basis for light-based electronic and photonic devices operating at PHz rates.

Ultracold atoms in optical lattices provide an optimal platform to study microscopic processes in quantum many-body systems. Theoretical models from solid state physics can be implemented in the laboratory and their predictions can be tested in novel ways. One of the recent advancements in the field is the realization of magnetic systems that can be manipulated on the level of single elementary magnets. Using this technique, fundamental magnetic excitations, so called magnons, and their dynamics and interactions have been recently observed.

An exotic type of hydrogen, where the central nucleus – a single proton – is orbited by a muon instead of an electron, permits novel studies of the proton structure. The surprising result is that the charge radius of the proton is 4% smaller than previously measured. This “proton radius puzzle” has led to many speculations, even questioning the theory of quantum electrodynamics or the Standard Model of particle physics. Scientists at MPQ hope to shed new light on the “proton radius puzzle” using laser spectroscopy of muonic deuterium atoms and muonic helium ions.

High energetic particles, for example ions play an important role in a variety of processes. Physicists in the Laboratory for Attosecond Physics investigate ion acceleration using laser pulses of very high intensity. Their research aims at evaluating this new technology with respect to its application and efficiency in ion therapy of tumours.
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Quantum networks allowing for the transfer and storage of quantum information will one day enable distributed quantum computing. An elementary version of such a quantum network based on single atoms was recently demonstrated. The atoms are perfectly suited for the storage of quantum states, which can be transferred between remote single-atom network nodes by the coherent exchange of single photons. The efficient interface between single atoms and single photons is based on an optical resonator and allows for the creation of entanglement between two network nodes.

A fundamental feature of quantum physics is the existence of superposition states. They are, e. g., created in a double slit experiment, where a particle passes through both slits of an interferometer at the same time to interfere with itself downstream. This spatially separated quantum superposition has been observed for particles ranging from electrons to complex molecules. But what happens for even more massive systems? The achievement of large superpositions of heavy objects should confront some conjectures that predict the breakdown of the quantum superposition principle at larger scales.
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Main topic of the Max Planck Research Group Ultrafast Quantum Optics, established in 2008 at the Max Planck Institute of Quantum Optics, is the control of free electrons with new methods. Recently the group succeeded in steering the emission of electrons from a metal nanotip by the application of ultrafast laser pulses. This is a first step towards the development of an attosecond field effect transistor. Using the technique of a "Paul trap" the group was able to capture free electrons by microwaves, and guide them along electrodes on planar substrates.
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An ensemble of ultracold atoms in an optical lattice can behave like a superconductor or like an insulator. Scientists have now managed to observe this behavior with a microscope – atom by atom, lattice site by lattice site. Moreover, they have succeeded in addressing individual atoms on the lattice sites and changed their quantum state. This offers great perspectives for quantum information processing.
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During the interaction of matter with the extreme light intensities occurring in the focus of a high-intensity laser, new and surprising effects take place. Especially in plasma, very strong fields are generated, which are long-lived in comparison with the driving field oscillations. Hence they are well suited for the collective acceleration of particle bunches, which display some unique properties which seem to make them ideally suited for driving new and ultrashort X-ray sources. However, the tight specifications for the driving laser pulses motivate a continuous advance in laser technology.
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According to modern cosmological theories, matter and antimatter were created in almost equal amounts after the Big Bang. But the visible universe now seems to be made entirely out of matter. Researchers at the Max Planck Institute for Quantum Optics used particle accelerators at the CERN laboratory of Geneva to synthesize exotic atoms containing antimatter particles. One of these is antiprotonic helium, which consists of an antiproton and electron orbiting a helium nucleus. Lasers were used to precisely study its properties and explore the fundamental symmetries between matter and antimatter.
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For decades physicists have been trying to realize a so-called phonon laser, a device that represents a mechanical analogue to an optical laser. It operates based on quantized oscillations (phonons) as opposed to quantized light (photons). This has now been demonstrated for the first time using a single, trapped ion. The crucial ingredient was a blue-detuned laser beam that serves as an energy source for the ion’s motion. Moreover, it could be shown that the phonon laser is a promising system for the detection of ultra-weak forces.
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Gases with temperatures close to absolute zero make it possible to study a plethora of quantum mechanical effects which could not be seen at higher temperatures because of the thermal motion of the particles. These effects include Bose-Einstein condensation, reversible chemical reactions as well as a variety of effects in optical lattices.
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The nuclei in atoms and molecules move typically on femtosecond timescales (1 fs = 10-15 s). Electrons are moving much faster and may change their position on timescales down to attoseconds (1 as = 10-18 s). New experimental techniques allow to take movies of the electrons on this timescale and to gain deeper insight into the ultrafast dynamics of electrons in atoms, molecules and nanostructures.
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Optical microcavities store resonant light for millions of round-trips. The high sensitivity of the resonance condition enables the direct observation of Brownian motion and the detection of biological molecules. The resonant power enhancement gives rise to novel nonlinear processes due to light forces exerted on the resonator as well as an extreme form of the Kerr nonlinearity, which transforms a continuous-wave laser into a frequency comb.
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A deeper insight into complex quantum systems requires computer simulations to improve by a large amount. This could in principle be done by a universal quantum computer, its experimental set up is however a decade away. For certain tasks, e.g. the simulation of quantum magnetism or high-temperature supraconductivity, an analog quantum simulator could be a short cut. A very promising concept is an ion trap with the ions serving as quantum bits.
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Only a few years after the first generation of single isolated attosecond pulses of
extreme ultraviolet light, these pulses can be produced routinely. First applications are
investigations of light and matter in which ultrafast processes are being time-resolved.
The direct measurement of the electric field of an ultrashort light pulse as well as the
time-resolved observation of a photoionization process in a gas or on a solid surfce show
the potential of attosecond physics.
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Quantum cellular automata provide possible realizations of quantum computers without the need of local addressing. The storage and processing of quantum information is then completely delocalized such that the system remains translationally invariant throughout. Possible physical implementations can be atoms trapped in optical lattices.
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Abstract
The production and investigation of cold molecules is a young and fascinating field of research. In our research group, electric fields were used to filter slow polar molecules out of a thermal gas. These molecules could be trapped in an electric cage for 300 ms where their temperature amounts to 300 mK.
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The frequency comb technology that was awarded the Noble Prize of Physics enables frequency measurements of highest accuracy and the development of high-precision “optical clocks”. New applications e.g. in geology as well as new discoveries concerning fundamental physical theories can be envisioned.
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The article gives a historical account of the activities of the Laser-Plasma-Group at the MPI for Quantum Optics (MPQ) including the period prior to the formation of the institute. The investigated objects of research have mainly dealt with fundamental issues of inertial fusion using laser beams and far less with questions related to the technical realisation of this concept.
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Chemistry is the science of materials changes and their associated dynamics. A firm grip on such intimate details is only possible by laser techniques. They provide detailed and basic insight into the dynamics of molecular and biomolecular changes on a molecular level. Both electronic and nuclear dynamics is of interest here. The following report is a summary of the work of the laser chemistry group at the MPQ Garching, extending to excitation and reactions of molecules in the electronic groundstate, i.e. on the initiation of purely nuclear motion. It is intended to pave the ground for more advanced electron dynamics in reactive chemical and biochemical systems. This, however, will require even higher time resolution – beyond the present age of femtochemistry.
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When quantum physics was developed, it was absolutely inconceivable that quantum processes of single atoms could ever be observed. Discussion of the subject was thus always confined to gedanken experiments and quantum mechanics simply made do with the conception that probabilities can only be stated for an ensemble. In recent years, however, the methods used in laser spectroscopy have made it possible to observe single free atoms and also single molecules in solids and, in particular, to trace their quantum behaviour. The ability to manipulate the vacuum field that comes into play in quantisation of the radiation field has afforded further interesting experiments. It is thus possible nowadays to trace the quantum processes and their external influence in single atoms and make phenomena visible that to not occur in the observation of several atoms or that would be averaged out. The fact that the vacuum field can be manipulated has yielded a new field of research named resonator quantum electrodynamics. The experiments conducted at MPQ in this area laid the foundation. They have led to the observation of a series of new results, which are summarised int he following
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The most modern microscopes allow research scientists to observe single atoms in their rest state. If, however, atoms are moving, very short light pulses are needed to allow the motion to be reconstructed from a series of snapshots. Whereas an exposure time of less than a thousandth of a second is sufficient to make a sharp image of a tennis ball in flight, the light pulses have to be shortened a billionfold to just a few femtoseconds in order to record the fastest atomic motions in molecules. Inside the electron sheath of excited atoms electrons move a thousand times as fast. They change from one energy state to another in a time of typically 10 to 1000 attoseconds. Atoms originally bound in a molecule then fly apart or transmit ultraviolet radiation or X-radiation. These processes are of fundamental importance for controlling chemical reactions and synthesising new materials. They might even be applied to designing a handy X-ray laser.
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